1. Field of the Invention
The present invention relates to a method for forming images by depositing finely-divided resin particles on the surface of an image-bearing member, and an image forming apparatus in which the above-mentioned image forming method is carried out.
2. Discussion of Background
The offset printing is conventionally utilized to obtain a small quantity of copied materials. A master plate for use in the offset printing is generally prepared by the following methods, but these methods have their own disadvantages:
(1) The master is prepared from a presensitized (PS) plate.
This method necessitates a light source of laser, so that the apparatus for preparing the master is made complicated. In addition, some chemical treatment is necessary at the final stage of the preparation of the master.
(2) The master is prepared in accordance with Electrofax in the field of electrophotography.
In this method, a latent electrostatic image is first formed on a recording member in which zinc oxide is dispersed. The latent electrostatic image thus formed is developed with a toner to obtain a visible toner image, and then toner-image portions or the background of the recording member is subjected to hydrophilic treatment to apply a printing ink thereto. This process for preparation of the master is thus also made complicated.
To solve the problem of the complicated process for preparing the master, the following master-preparation methods are proposed:
(3) The master is prepared by attaching an ink sheet to a sheet-shaped base material for the master, and selectively applying heat to the ink sheet to thermally transfer the ink component in the ink sheet to the base material (Japanese Laid-Open Patent Applications 54-143303, 63-60752 and 2-217247).
According to this method, the master is available in the form of a sheet. Therefore, once the sheet-shaped master is subjected to the printing, it cannot be repeatedly used. Consequently, the printing cost is increased.
(4) The master is prepared by transferring an ink to the base material for the master, the ink to which the viscosity can be imparted depending upon the change of the pH value thereof (Japanese Laid-Open Patent Application 1-166959).
In this method, the printing speed cannot be increased because the ink transferred to the base material for the master has relatively high viscosity.
Furthermore, the following image forming methods, that is, the master preparation methods, are conventionally proposed:
(5) The master is prepared by depositing a thermofusible ink in the form of particles on an image-bearing member with the application of heat thereto to form images thereon. The thus formed images are transferred to a recording sheet by the application of heat or pressure thereto (Japanese Laid-Open Patent Application 62-279966).
This method has the shortcomings that the scattering of ink often occurs in the apparatus because the ink is used in the form of particles, and that it is difficult to prevent the ink deposition on the background because the ink is easily attached to a non-image area by the electrostatic force.
(6) The master is prepared by bringing a liquid-type ink capable of increasing the viscosity upon application of heat thereto into contact with an ink-repellent image-bearing member, and selectively applying heat thereto, thereby forming ink images on the image-bearing member. Then, the ink images are transferred to a recording sheet (Japanese Laid-Open Patent Applications 3-227255 and 4-45955).
The image forming method (6) has the following disadvantages:
(i) It is necessary to provide convex and concave portions on the surface of the ink-repellent image-bearing member. Therefore, the cleaning of the surface of the image-bearing member is difficult, and consequently, the image quality is impaired.
(ii) In the case where the viscosity of the liquid-type ink is increased by causing a solvent component therein to evaporate by the application of heat thereto, the evaporation of the solvent necessitates a relatively long period of heating time.
(iii) In the case where the viscosity of the liquid-type ink is increased by containing in the ink a material of which solubility decreases with the elevation of temperature, a relatively long period of heating time is required.
(iv) In the case where the viscosity of the liquid-type ink is increased by causing gelation of the ink upon application of heat thereto, moderate heating is required to cause the gelation at a relatively low temperature, which necessitates a long period of heating time.